1. Field of the Invention
The present invention relates to the retorting of oil shale in a staged turbulent bed with raw shale preheat.
2. Description of the Prior Art
Vast natural deposits of shale in Colorado, Utah and Wyoming contain appreciable quantities of organic matter which decompose upon pyrolysis to yield oil, hydrocarbon gases and a carbonaceous residue. The organic matter or kerogen content of these deposits has been estimated to be equivalent to approximately 500 million cubic meters of oil. As a result of the dwindling supplies of petroleum and natural gas, extensive research efforts have been directed to develop retorting processes which will economically produce shale oil on a commercial basis from these vast resources.
In principle, shale retorting simply comprises heating the raw shale to an elevated temperature and recovering the vapors evolved. The process heat requirements may be supplied either directly or indirectly. Directly heated retorting processes rely upon the combustion of fuel in the presence of the oil shale to provide sufficient heat for retorting. Such processes result in lower product yields, due to unavoidable combustion of some of the product, and dilute the product stream with the products of combustion. Indirectly heated retorting processes generally use a separate furnace or equivalent vessel in which a solid or gaseous heat carrier is heated. The hot heat carrier is subsequently mixed with the shale to provide heat, thus resulting in higher yields while avoiding dilution of the retort products with combustion products.
As used herein, the term "oil shale" refers to fine-grained sedimentary material which is predominantly clay, carbonates and silicates in conjunction with organic matter composed of carbon, hydrogen, sulfur, oxygen, and nitrogen, called "kerogen".
The term "retorted shale", as used herein, refers to oil shale from which essentially all of the volatizable hydrocarbons have been removed, but which may still contain carbonaceous residue.
The term "spent shale", as used herein, refers to retorted shale from which a substantial portion of the carbonaceous residue has been removed, for example by combustion in a combustion zone.
The terms "condensed", "noncondensable", "normally gaseous", or "normally liquid" are relative to the condition of the subject material at a temperature of 25.degree. C. (77.degree. F.) and a pressure of one atmosphere.
Particle size, unless otherwise indicated, is measured with respect to Tyler Standard Sieve sizes.
In the staged turbulent bed retorting process, crushed raw oil shale particles are introduced by conventional means, into an upper portion of a retort, and passed downwardly therethrough. Spent shale, at an elevated temperature, is also introduced by conventional means into the upper portion of said retort and passes downwardly therethrough cocurrently with the raw crushed oil shale. The maximum particle size for the shale introduced is maintained at or below 21/2 mesh, Tyler Standard Sieve size. Particle sizes in this range are easily produced by conventional means such as cage mills, jaw, or gyratory crushers. The crushing operations can be conducted to produce a maximum particle size, but little or no control can be effected over the smaller particle sizes produced. This is particularly true in regard to the crushing of shale which tends to cleave into slab or wedged-shape fragments.
The temperature of the spent shale introduced to the retort will normally be in the range of 600.degree. C.-800.degree. C., and an appropriate operating ratio of hot spent shale to raw shale may be selected to maintain the temperature at the top of the retort in the broad range, 450.degree. C. to 540.degree. C., and preferably in the range of 480.degree. C. to 510.degree. C.
The weight ratio of spent shale heat carrier to raw oil shale may be varied from approximately 1.0:1 to 8:1, with a preferred weight ratio in the range of 2.0:1 to 3:1. It has been observed that some loss in product yield occurs at the higher weight ratios of spent shale to fresh shale and it is believed that the cause for such loss is due to increased adsorption of the retorted hydrocarbonaceous vapor by the larger quantities of spent shale. Furthermore, attrition of the spent shale, which is a natural consequence of retorting and combustion of the shale, occurs to such an extent that high recycle ratios cannot be achieved with spent shale alone. If it is desired to operate at the higher weight ratios of heat carrier to fresh shale, an auxiliary attrition resistant material, such as sand, must be substituted as part or all of the heat carrier.
The mass flow rate of fresh oil shale through the retort should be maintained between 4,900 kg/hr-m.sup.2 and 29,300 kg/hr-m.sup.2, and preferably between 9,800 kg/hr-m.sup.2 and 19,600 kg/hr-m.sup.2. Thus, in accordance with the broader recycle heat carrier weight ratios stated above, the total solids mass rate will range from approximately 12,200 kg/hr-m.sup.2 to 261,000 kg/hr-m.sup.2.
A stripping gas, preferably steam, is introduced into a lower portion of the retort and passes upwardly through the vessel in countercurrent flow to the downwardly moving shale. The flow rate of the stripping gas should be maintained so as to produce a superficial gas velocity at the bottom of the vessel in the range of approximately 30 cm per second to 150 cm per second, with a preferred superficial velocity in the range of 30 cm per second to 60 cm per second. The stripping gas may be comprised of steam, recycle product gas, hydrogen or any inert gas. It is particularly important, however, that the stripping gas selected be essentially free of molecular oxygen to prevent product combustion within the retort.
The stripping gas will fluidize those particles of the raw oil shale and spent shale heat carrier having a minimum fluidization velocity less than the velocity of the stripping gas. Those particles having a fluidization velocity greater than the gas velocity will pass downwardly through the retort, generally at a faster rate than the fluidized particles.
Limiting the vertical backmixing of the downwardly moving shale and heat carrier by mechanical means produces stable, substantially plug flow conditions throughout the retort volume. True plug flow, wherein there is little or no vertical backmixing of solids, allows much higher conversion levels of kerogen to vaporized hydrocarbonaceous material than can be obtained, for example, in a fluidized bed retort with gross top to bottom mixing. In conventional fluidized beds or in stirred tank type reactors, the product stream removed approximates the average conditions in the conventional reactor zone. Thus, in such processes partially retorted material is necessarily removed with the product stream, resulting in either costly separation and recycle of unreacted materials, reduced product yield, or a larger reactor volume giving much longer average particle residence times. Maintaining substantially plug flow conditions by substantially limiting top to bottom mixing of solids, however, allows one to operate the process on a continuous basis with a much greater control of the residence time of individual particles. The use of mechanical means for limiting substantial vertical backmixing of solids also permits a substantial reduction in size of the retort zone required for a given mass throughput, since the chances for removing partially retorted solids with the retorted solids are reduced. The mechanical means for limiting backmixing and limiting the maximum bubble size may be generally described as barriers, baffles, dispersers or flow redistributors, and may, for example, include spaced horizontal perforated plates, bars, screens, packing, or other suitable internals.
Gross vertical backmixing should be avoided, but highly localized mixing is desirable for purposes of heat transfer in that it increases the degree of contacting between the solids and the solids and gases. The degree of backmixing is, of course, dependent on many factors, but is primarily dependent upon the particular internals or packing disposed within the retort.
A product effluent stream comprised of hydrocarbonaceous material admixed with the stripping gas and some entrained fines is removed from the upper portion of the retort by conventional means.
The retorted shale along with the spent shale serving as heat carrier is removed from the lower portion of the retort by conventional means at the retort temperature and fed to a lower portion of a combustor.
While the combustor may be of conventional design, it is preferred that it be a dilute phase lift combustor. Air is injected into the lower portion of the combustor and the organic residue on the shale is burned. The combustion heats the retorted shale to a temperature in the range of 600.degree. C. to 800.degree. C. and the hot shale and flue gas are removed from the upper portion of the combustor. A portion of the hot spent shale is recycled to provide heat for the retort. Preferably said recycled shale is classified to remove substantially all of the minus 200 mesh shale fines prior to introduction to the retort in order to minimize entrained fines carryover with the effluent product vapor.